Resistor spark plugs employ a glassy, relatively high resistance seal material between the terminal screw and the center electrode. During spark plug manufacture, such a seal composition is added as a particulate mixing to the center bore of an insulator body onto the upper end of a previously placed center electrode. A metal terminal screw is then placed in the bore of the insulator so that its lower end rests on top of the particulate composition. The assembly is then fired in a furnace at a relatively high temperature to fuse the glass and soften the material so that the terminal screw can be pushed down into the fused composition.
The firing of the composition produces a fused glassy mass that provides a gas-tight seal in the interior of the spark plug insulator body between the center electrode and the terminal screw. The composition contains metal particles which during the firing operation fuse and provide a bond between the metal conductors and the resistive seal composition.
Metallized glass seal resistor compositions such as those disclosed in U.S. Pat. No. 4,112,330 and U.S. Pat. No. 4,795,944 serve a useful function in the operation of a spark plug in an automobile engine. The high resistance material within the spark plug suppresses the generation of radio frequency electromagnetic radiation which would interfere with radio reception in nearby vehicles. In performing this important function, it is important that the original particulate mixture fuse upon firing to form a mass that has a predictably high level of resistance and that such level of resistance not change appreciably during prolonged usage of the spark plug in engine operation.
The glass metal compositions disclosed in the above-identified U.S. patents have functioned well in millions of spark plugs in automobile engines. Specifically, with the design of more sophisticated automotive ignition systems, the composition disclosed in U.S. Pat. No. 4,795,944 is available which will predictably fire to form resistors with a more narrow resistance range. The formed seals disclosed in U.S. Pat. No. 4,795,944 also maintain resistor values close to their as-fired condition throughout prolonged engine operation.
In the processing of metallized glass seal resistor composition, it has been noticed that since barium borate glass causes an exothermic reaction when water is added during the wet mixing and granulation process, it is frequently necessary for production personnel to test each lot of glass for its heat-up characteristics, i.e., mixing time and temperature. As a consequence, it is frequently necessary to blend various lots of glass together to achieve a constant temperature. The process is, therefore, operator sensitive and requires a great deal of control to make the glass batches consistently.
It is, therefore, an object of the present invention to provide a metallized glass seal resistor composition that enables a gas-tight seal in the interior of the spark plug insulator body between the center electrode and the terminal screw.
It is another object of the present invention to provide a metallized glass seal resistor composition which will predictably fire to form resistors with a more narrow resistance range to suppress the generation of radio frequency electromagnetic radiation.
It is a further object of the present invention to provide a metallized glass seal resistor composition that can be easily processed without requiring a great deal of operator control to achieve batch consistency.
It is yet another object of the present invention to provide a metallized glass seal resistor composition that has lower softening temperature to provide for a more homogeneous composition upon sealing and an improved resistance uniformity.